Hungary was attaching razor wire to a fence erected at its border with Croatia, in a possible prelude to sealing the frontier to thousands of migrants.
Hungarian soldiers, wearing thick leather gloves, were unrolling the wire and attaching it at ground level to the fence near the Beremend crossing point in Hungary's south, AFP reporters said.
Next to the road where people cross, there were two metal gates two metres high and three metres high on wheels that can be rolled into place and block entry at any point.
Hungary last month sealed its border with Serbia, cutting off the main entry point for tens of thousands of migrants travelling up through the western Balkans trying to make it to northern Europe.
This diverted the flow of migrants, mostly from Syria, Iraq, Afghanistan and other hotspots, into Croatia. But Zagreb has since been transporting them to the Hungarian border, and Hungary takes them to the Austrian frontier.
At Beremend, Hungarian authorities have also set up a "transit zone" made up of 25 dark blue containers lined up next to the two-lane road that runs from Croatia to Hungary.
Another fence, about two metres away, forms a path to the "transit zone" where Hungarian authorities plan to examine - and likely reject, rights groups say - requests for asylum once it is operational.
About 200 kilometres northwest of Beremend, at the border post of Letenye, a similar transit zone of 28 containers has been put in place.
Hungarian Prime Minister Viktor Orban last week said he aimed to close the border with Croatia but that he would discuss the move with UN chief Ban Ki-moon at this week's UN General Assembly in New York.
A Hungarian government spokesman, Zoltan Kovacs, told AFP that "the border issue" would be discussed at a cabinet meeting on Friday (local time).